Obtain and purify lymphocyte activating factor (LAF) from normal human lymphocytes and test its mitogenic action on mouse thymocytes. Examine LAF production; identify better conditions for its production. LAF concentrates of the highest purity shall be used in a survey of its biological targets, with examination of a variety of murine and human cells. A detailed study shall be made of the site of action of LAF in early steps of cell triggering, using a battery o of pharmacologic agents. LAF shall also be tested in vivo for its effect on the immune response to weakly antigenic tumors.